A Phytophthora sojae Heat Shock Transcription Factor PsHSF1 is Required for Oxidative Stress Tolerance and Suppression of Plant Immunity

Although much is known about that reactive oxygen species (ROS) play central roles in plant defense system, the mechanisms how phytopathogens adapt and counteract the plant-driven ROS remain little understood. Genome-wide analysis of ROS tolerance-related transcription factors in Phytophthora species uncovered an extremely huge gene family of heat shock transcription factors (HSFs), 22 HSF genes in P. sojae, 24 HSF genes in P. infestans and 18 HSF genes in P. ramorum, while only 1-4 HSFs in true fungi, such as Magnaporthe or yeast. A highly up-regulated P. sojae HSF under oxidative stress condition, PsHSF1, was given a detailed study, which was also strongly up-regulated under heat shock stress and at cyst germination stage. PsHSF1 silencing reduced the rate of cyst germination. Hyphal growth of PsHSF1- silenced transformants was severely affected under the stress of hydrogen peroxide or heat shock compared with wide-type (WT). The soybean infection of PsHSF1-silenced transformants was significantly delayed. Cytological staining showed that the mutants compromised to suppress hydrogen peroxide accumulation and innate immunity of soybean cell around the infection area. The results demonstrate that PsHSF1 is required for the adaptation and response to stress mediated with ROS in P. sojae, and suggest that PsHSF1 played an important role in counteracting ROS-mediated plant innate immunity.